Variable selectivity amplifier



Deco-1 1, 1945.

K. B. AUSTlN ETAL VARIABLE SELECTIVITI AMPLIFIER Filed Oct; '7, 1944 RAD/O SECOND FREQUENCY DE TE C TOR AMPLIFIER All/D All/D UT/L/ZAflU/V CONVERTER DEV/CE Tgzy Fi g2.

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x I k k k l w I I FREQUENCY FREQUENCY FREQUE/YC Y I r-wventow 5: Kirby 15. Austin,

' George G.Young., V

Their" Attorney.

Patented Dec. 11, 1945 umrsp STATES PATENT ascents OFFICE VARIABLE SELECTIVITY Application October 7, 1944, Serial No. 557,.tt9

This invention relates to" high frequency ampllfiers and particularly to such amplifiers which are provided with. switching devices for selectively changing the resonance bandwidth.

It is an object of this invention to provide a variable selectivity high frequency amplifier including an improved arrangement for changing the resonance bandwidth of the amplifier. v

It is another object of this invention to provide a variable selectivity high frequency amplifier including an improved bandwidth selection arrangement of simplified construction. v

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of the invention, and Figs. 2, 3, 4, and

8 Claims. writs-171) Referring now to the drawing. the amplifier shown in Fig. 1 includes first and second stage electron discharge devices I and 2, respectively.

represent certain operating characteristics of the apparatus of Fig. 1.

Briefly the apparatus illustrated in the drawing comprises a two-stage intermediate frequency electronic amplifier provided with tuned circuits directly coupled through suitable reactance elements, one pair of tuned circuits being arranged in the input of the first stage of the amplifier and a second pair of tuned circuits in the output thereof. The coupling reactances are associated with the primary tuned circuits of each pair and are of opposite sign; that is, one is inductive and the other is capacitive. A switching arrangement is provided for simultaneously changing the coupling of the secondary circuits from the position including only a portion of the reactance elements to the position including all of the reactance elements, together with an additional resistance element included only in the secondary circuits. The values of the reactances are selected so that when the units are tuned to resonance with only portions of the elements common to the two circuits, the changing of the control to include the entire elements provides a wide resonance bandwidth, one of the secondary circuits being detuned below the resonance frequency and the other above the resonance frequency so that the combined or overall characteristic is symmetrical about the resonance frequency. The voltage gain for the two positions of the control is made substantiallycqual so that no adjustment of the gain is neccssary.

The device I is coupled to the output of a radio frequency amplifier and converter 3 through a pair of tuned circuits 4 and 5. The circuit a is the primary circuit and includes a main inductance 8 and a main capacitance l. The secondary circuit 5 includes a main inductance 8 and a main capacitance 9. and the two circuits are directly coupled through an inductive reactance element I which is connected in series with the main inductance it of the circuit s. In the position shown the direct coupling is effected through the left-hand contact of a double-throw switch it so that only a portion of the reactance III is common to both circuits. The main inductance elements 8 and t are provided with movable cores l2 and M respectively for tuning purposes, and these circuits are tuned to resonance at a predetermined frequency when switch It is in its left-hand position as shown. The circuit 5 1s connected across-a resistor ll to a control grid it of the device 9, which also includes an anode it and a cathode it. The devices I and 2 are coupled by a pair of tuned circuits l8 and it. The circuit It includes a main capacitor 29 and a main inductance M which may be tuned by movement of a core 22. The circuit 19 includes a main capacitance 23 and a main inductance 24 which may be tuned by movement of a core 25. A capacitive reactance element comprising ca:

'pacitors 26 and 2? connected in series with the main capacitor it is provided for coupling the circuits l8 and I8, and the circuit i9 is connected in common with the capacitor 21 through the left-hand contacts of the gouble-throw switch 28. A suitable source ofano' e voltage indicated as a battery 28 is provided and is connected to the plate It through the inductance 2| and a resistor 30. The battery 20 is also connected through the inductances 6 and ill to supply the anode voltage for the last tube in the amplifier and converter 3.

The pair of circuits l! and i9 is similar to the pair 4 and 5 except that the direct coupling is capacitive rather than inductive, and the circuits I l and II are tuned to resonance at the same predetermined frequency when the switch 20 is in its left-hand position. The output of the circuit I0 is connected'to a control electrode it of the device 2 which amplifies the output and supplies it to a. suitable utilization device 32 connected between the anode 33 and the cathode 84 of device 2.

The amplifier of Fig. 1 when operated with the switches H and 28 in their left-hand position is sharply tuned and highly selective. Under certain conditions, however, it is desirableto connect the amplifier to operate with a broad resonance bandwidth. This may be accomplished by moving the switches II and 28 to their righthand positions. To facilitate the simultaneous movement of the switches, they may be connected by a suitable mechanical coupling indicated by a dotted line 35. When the switches are in their right-hand positions, the entire reactance elements I!) and 23, 21 are connected in common with their respective primary and secondary circuits. Furthermore a resistance 35 is connected in series with the reactance ill in the circuit 5, and a resistance 31 is connected in series with the condensers 26 and 21 in the circuit l9. These resistors reduce the eifective ratio of the energy stored in the resonance circuits to the energy dissipated therein, and thus Y tive. The values of the portions of the coupling.

reactances connected in the secondary circuits may be selected so that the voltage gain, that is ratio of the input voltage to the output voltage, of each pair of circuits remains the same for both positions of the double-throw switches. That this may be accomplished is illustrated in Fig. 2 where the mutual coupling curve for the pair of circuits 4 and 5 is illustrated. It will be noted that the curve has a hump 31 which has a maximum value of gain at the point of critical coupling where the transfer of energy is maximum. By selecting two points 38 and 39 respectively below and above the critical couplingvalue, it is possible to provide two points of equal gain with different values of coupling and the values of the two portions of the reactance l are selected accordingly, the value of the portion of the reactance connected when the switch II is in its left-hand position being that at 38 and the value when the entire reactance is included in the circuit and the switch II is in its right-hand position being that at 39. A similar curve may be drawn to illustrate the variation of gain with mutual coupling for the pair of circuits l8 and I8, the values of the condensers 26 and'21 being selected to provide equal gain in a manner similar to the selection of the two portions of the inductance l0.

The coupling between the primary and secondary circuits of each pair may be made entirely dependent upon the impedance of the coupling element by electromagneticallyand electrostatiand which are selectivity curves, the frequency above and below the mean resonant frequency being plotted against attenuation. In each of these figures, the sharp resonance characteristic curve is indicated at 42, this being the desired sharp resonance curve with the switches in their left-hand position and at a frequency indicated by the dotted line 43. Fig. 3 represents at 44 the selectivity curve of the pair of circuits 4 and 5 when the switch II is in its right-hand position,-

the secondary circuit 5 producing detuning in the range'of frequencies below the frequency 43.

Fig. 4 represents at 45 the selectivity curve of the circuits l8 and I9 when-the switch 28 is in its right hand position, this curve showing that the secondary circuit is detuned for.frequencies above the frequency 43 due to the capacitive coucurve 46 is a broad over-all resonance curve resulting from the combined operating characteristics of the two pairs of tuned circuits 4, 5 and l8, l9.

After selecting the desired values of the couplingreactances I0 and 26, 21, the circuit of Fig.

1 is prepared for operation by placing the switches II and 28 in their left-hand positions as shown in the drawing and. then tuning the circuits 4, 5, l8, and 19 to resonance at the de- 30 sired predetermined frequency 43. This simple tuning operation is the only tuning required, it being unnecessary to make anyfurtheradjustments of the tuning when the coupling is changed to place the switches I l and 28 in ,their 35 right-hand positions.

It is readily apparent from the foregoing that a simple and eflective circuit has been provided for securing either a broad or a sharp resonance bandwidth in a high frequency amplifier and that a minimum number of circuit components are required. Furthermore only a single tuning operation is necessary for bothpositions of the bandwidth selecting device. Furthermore all the switching may be effected on the low voltage endof the tuned circuits and it is unnecessary to I shield the switching mechanism. Furthermore this circuit makes it possible to provide constant gain for both positions of the selectivity changing device. By way of illustration only and not by way of limitation, there are listedbelow values of circuit constants which have been found to be suitable for the circuit of Fig. 1 when employed in a particular intermediate frequency amplifier. The electronic discharge devices I and 2 were 12sG7 pentodes, and the circuits 4, a. la, and I3 were tuned to an intermediate frequency of 1225 kilocycles when the switches I l and 28 were in their left-hand positions. The other values were I as follows:

While the invention has been illustrated in connection with an intermediate frequency amplifier for a superheterodyne receiver, other applications will readily be apparent to those skilled in the art. Therefore, it is not desired that the invention be limited to the specific circuit arrangement described and illustrated, and it is intended by the appended claims to cover all modiacas;

tions within the spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A variable selectivity high frequency ampli fier comprising in combination, first and second pairs of tunable circuits, means for interconnecting said pairs of circuits, a reactance of one sign connected in one circuit of said first pair and having a portion common to the other circuit of said first pair for coupling said circuits of said first pair, a reacta'nce of the opposite sign connected in one circuit of said second pair and having a portion common to the other circuit of said second pair for coupling said circuits-of said sec- (and pair, said circuits being tuned to a predetermined resonance frequency, and selectively operable means for increasing the portions of said rcactances common to said other circuits whereby means movable into one position for connecting the other circuit of each of said pairs across a predetermined portion of the corresponding one -of said elements in the pair and movable into a means is in said first position whereby in said 2. A variable selectivity high frequency amplifier comprising in combination, first and second pairs of tunable circuits, means for interconnecting said pairs of circuits, one of the circuits of each of said pairs including a reactance element, the one of said reactance elements in one pair being inductive and that in the other pair being.

capacitive, each of said elements having a portion connected in the other circuit of its respective pair for directlycoupling the circuits in each second position said other circuits are detuned oppositely to provide a broad over-all resonance bandwidth.

5. A variable selectivity high frequency amplifier comprising in combination, first and second pairs of tuned circuits, each comprising a primary tuned circuit and a secondary tuned circuit, means for interconnecting the secondary circuit of said first pair and the primary circuit of said second pair, each of said pairs of circuits including a reactance element connected in the primary circuit and having a portion thereof common to both the primary and secondary circuits for directly coupling the primary and secondary circuits of each pair, one of said reactance elements being an inductance and the other a capacitance, and selectively operable means for varying said common portions of said reactance elements ing said pairs of circuits, one of the circuits of each of said pairs including a reactance element, the one of said reactance elements in one pair being inductive and that in the other pair being capacitive, each of said elements having a portion connected in the other circuit of its respective pair for directly coupling the circuits in each tially constant ratio of input voltage to output voltage for both pairs of circuits in both positions of said selectively operable means.

4. A variable selectivity high frequency amplifier comprising in combination, first and second pairs of tunable circuits, means for interconnecting said pairs of circuits, each of said circuits including a main, inductance anda main capacitance, means for electromagnetically and electrostatically shielding the main inductance and ,main

capacitance of each of said circuits, an inductive reactance element and a capacitive reactance element arranged outside said shielding means, means for connecting one of said elements in one circuit of said first pair, means for connecting the other of said elements in the corresponding circuit of said second pair, and selectively operable capacitive, each of said elements, having a portion connected in common with the other circuit of its respective pair for directly coupling the circuits in each pair, said circuits being tuned to resonance at a predetermined frequencyya resistance element associated with each of said pairs of circuits, and selectivel operable means for increasing the portions of said reactance elements common to said other circuits and for inserting the respective ones ofsaid resistance elements in said other circuits whereby both said pairs of circuits are overcoupled and their resonance bandwidths are broadened and said other circuits are detuned in opposite directions to provide an over-all bro'ad resonance bandwidth.

7. A variable selectivity high frequency amplifier comprising in combination, first and second pairs of tunable circuits, each comprising a primary circuit and a secondary circuit, means for interconnecting the secondary circuit of said first pair and the primary circuit of said second pair, each of said primary circuits including a reactance element, the reactance element in one of said primaries being inductive and that in the other capacitive, and selectively operable coupling means movable into one position for connecting a predetermined portion of each of said elements in common with the secondary circuit of its respective one of said pairs and movable into a second position for connecting a greater portion of each of said elements in common with the respective secondary circuit, said circuits being tuned to a predetermined frequency when said selectively operable means is in said one position whereby said oneposition provides a sharp resonance bandwidth and said second position provides a broad resonance bandwidth, the values of the common portions of said reactance elements being selected to provide substantiallyv equal ratios of output to input voltage for both pairs of circuits in both positions or said selectively operable means.

8. A variable selectivity high irequency amplifler comprising in combination, first and second I pairs of tunable gircuits, each of said pairs including a primer? circuit and a secondary circuit, means for interconnecting the secondary circuit of said first pair and the primary circuit of said second pair, each of said circuits including a main inductance and a main capacitance,

means for electromagnetically and electrostatically shielding the main inductance and main capacitance of each of said circuits, an inductive reactance element connected in series with the main inductance of one of said primary circuits, a capacitive reactance element connected in series with the main capacitance of the other of said primary circuits, both or said elements being arranged outside said smeldin'smeans. and selectively operable switching means arranged outside said shielding means and movable into a first position for connecting a predetermined portion 01. each 0! said elements in common with the secondarycircuit 01' the respective pair and movresonance bandwidth.

Y 3. sperm. GEORGE o. YOUNG. 

